Modern technology has produced a wide variety of electrical heating devices for use in cooking appliances. One class of electrical heating device which has been popular since the advent of readily available and economical electrical power has been the resistive electrical heating element.
Resistive electrical heating elements have been fabricated in a plurality of shapes over the years in an attempt to achieve the ideal heating element which is an element which will provide a flat surface having a uniform heat characteristic without exhibiting a gradient phenomenon or hot spots. This ideal heating element has been approached in electrical resistive heating but never actually accomplished in cooking appliances. For instance, E. Grahm in U.S. Pat. No. 3,798,415 on "Electrically Heated Cooking Utensil" issued Mar. 19, 1974, discloses the concept of a heating element which includes a heat transfer sink having a varying dimension calculated to shorten cooking time by increasing the heat conduction from the peripheral elements to the center of the appliance. This crudely approaches the concept of a uniform heat surface but the actual resistive heating elements are located in predetermined areas beneath the cooking surface and therefore a heat gradient will always exist across the cooking surface and hot spots in the areas of the resistive elements will occur.
D. Harris in U.S. Pat. Nos. 3,351,742 and 3,383,497 on "Electrical Resistance Heaters" issued Nov. 7, 1967 and May 14, 1968 teaches the concept in a graphite heating element of providing a graded thickness to the element so that the electrical resistance characteristics of the heater will be uniform and a roughly uniform heat will be produced across the surface of the element. A plurality of holes are drilled through the Harris heating element to ensure that approximately uniform heat distribution is achieved. Incorporating the plurality of holes, in the heating element of Harris, causes heat flow variations which are chosen experimentally to cause the heating element to approach a constant temperature surface, however the heat transfer characteristics created by the holes drilled in the element result in undesired variations in the surface temperature and thus the ultimate goal of a uniform temperature surface has been approached but not achieved. Hole distribution is wholly empirical and rests on no theoretical base.
L. Orr in U.S. Pat. No. 2,569,773 on "Electroconductive Article" issued Oct. 2, 1951, discloses the concept of providing an electrode having a varying thickness so that heat generated by the electrode is approximately uniform across the electrode surface, see for instance FIG. 5. Orr teaches the concept of a sprayed-on electrode on a window; the heat generated is utilized to defrost or deice a windshield. The concept, involving low energy input rates, is not applicable to cooking elements. Current flow is non-radial.
E. Thompson, U.S. Pat. No. 1,072,503 on "Electric Heater" issued Sept. 9, 1913, is a very early attempt to achieve predetermined heat characteristics by utilizing a heat transfer medium which is configured in varying thicknesses calculated to cause the actual heat transfer from an electrical resistive heating element to be conducted to a surface and radiated therefrom in an approximate uniform fashion. This concept is similar in basic principle to the heat transfer medium concept presented in Grahm previously discussed.
The concept of a tapered or varying thickness heat transfer medium is also incorporated in the E. Wolcott U.S. Pat. No. 1,485,153 on "Electric Heater" issued Feb. 26, 1924. In this embodiment, as in all other embodiments suggested, a significant drawback exists in that a special heat transfer means must be incorporated in the heating element and the uniform heat transfer of the device is only roughly approached. Variations in individual heaters cannot be accommodated by a mass produced heat transfer element and the market will not bear the added cost required to individually tailor each heat transfer element to match the characteristics of its associated resisting heating element. Again, there is no firm theoretical design foundation.
W. Hadaway Jr., U.S. Pat. No. 563,032 on "Electric Heater" issued June 30, 1896, is a very early example of an attempt to create a uniform heating surface in a cooking utensil. This device uses a spiral of coils having a decreasing diameter as the overall electrode spirals towards the center of the heating element. This approach results in a spirally shaped hot spot which decreases in temperature variation as the center of the element is approached and thus the goal of a uniform heating surface without hot spots or gradient is not achieved. The element is awkward, massive, and complex, with no quantitative design basis.